This invention relates to an improvement in making gelled hydrocarbons, especially those used in hydraulic fracturing of subterranean formations to enhance oil and gas production.
Hydraulic fracturing is a widely used method for stimulating oil and/or gas production. In performing a fracturing operation, a hydraulic fluid, usually a liquid hydrocarbon, is pumped into the well bore at sufficiently high pressure to fracture the surrounding rock formation to open cracks in the formation through which oil and/or gas can flow into the well bore. Since the cracks which are opened by the fracturing fluid tend to close once the pressure on the fluid is released, it is customary to inject into the well along with the fracturing fluid a suitable particulate proppant material such as sand. The small proppant particles flow into the fractures created in the formation along with the fracturing fluid and serve to prop the fractures open after the fluid pressure is released.
Proppant materials may be either lighter or heavier than the hydraulic fracturing fluid and thus may tend to float or settle out of the fluid prematurely, or otherwise be unevenly distributed in the fracturing fluid. To overcome this problem, it is customary practice to use gelled fracturing fluids which will hold the proppant material in suspension as the fluid flows down the well bore and out into the formation fractures. This requires that the gelled fracturing fluid be of sufficient viscosity to hold the proppant material suspended in a slurry or matrix. At the same time, the fluid must not be so viscous that it cannot be readily pumped into the well bore and out into the formation fractures.
Various materials are known which, when admixed with hydrocarbons, will create hydrocarbon gels of various viscosities. However, many of these materials are not suitable for use as hydrocarbon fracturing fluids because of the particular requirements imposed by the environment in which they are used. The gels must be formed at or near the wellhead at ambient temperature. Generally, several thousand gallons of normally liquid hydrocarbon such as crude or refined oil, a gelling agent and an activator are blended to form a gel. This mixture then is stored in frac tanks until used. Due to various factors such as the amount of fracturing fluid needed, labor schedules and other outside influences, the gel may be used promptly or a substantial period of time may elapse between the time the gel is initially formed and the time it is used. Therefore, it is desirable that the gel thicken relatively quickly to a sufficient viscosity to suspend the proppant material, but at the same time have stability over as long a period of time as possible so that it neither breaks down into a less viscous fluid, nor sets into a gel of such high viscosity that it cannot be pumped into the formation, before it is used.
Additional complications are presented by the fact that, while the gel is formed at ambient temperature at the wellhead, it is utilized in deep formations where temperatures may be much higher. Depending upon the depth, formation temperatures may be in the range of 200.degree. to 250.degree. Fahrenheit or higher. Such elevated temperatures tend to reduce the viscosity of the gelled liquid. Prior art gelled hydrocarbons which are of sufficient viscosity to suspend the proppant, but not too viscous to pump, generally will not retain sufficient viscosity at temperatures of 200.degree. to 250.degree. or more to retain the proppant in suspension.
An ideal gelling agent for forming a gelled hydrocarbon fracturing fluid would be one which, when mixed with the normally liquid hydrocarbon and an activator, relatively quickly forms a gel of sufficient viscosity to hold the proppant in suspension and then retains its desired range of viscosity for at least several hours at ambient temperatures. At the same time, the gel should, when injected into downhole formations at temperatures of 200.degree. to 250.degree., retain sufficient viscosity to keep the proppant in suspension for the period of time required for the fracturing operation, which may be up to eight hours. It is accordingly the primary object of the present invention to provide an improved gelling agent, hydrocarbon gel, and methods of making and using the same, which meet these criteria.